[req-change] Return "unspecified" when openwisp installation method i…

…s not defined

openwisp_utils/admin_theme/system_info.py

@@ -17,19 +17,19 @@ def get_installed_openwisp_packages():
     }
 
 
-def _get_openwisp2_detail(attribute_name):
+def _get_openwisp2_detail(attribute_name, fallback=None):
     try:
         return import_string(f'openwisp2.{attribute_name}')
     except ImportError:
-        return None
+        return fallback
 
 
 def get_openwisp_version():
     return _get_openwisp2_detail('__openwisp_version__')
 
 
 def get_openwisp_installation_method():
-    return _get_openwisp2_detail('__openwisp_installation_method__')
+    return _get_openwisp2_detail('__openwisp_installation_method__', 'unspecified')
 
 
 def get_enabled_openwisp_modules():

openwisp_utils/measurements/tests/test_models.py

@@ -7,6 +7,7 @@
 from django.test import TestCase, override_settings
 from freezegun import freeze_time
 from openwisp_utils import utils
+from openwisp_utils.admin_theme import system_info
 from urllib3.response import HTTPResponse
 
 from .. import tasks
@@ -36,6 +37,10 @@ def setUp(self):
             ).replace(tzinfo=timezone.utc),
         )
 
+    @patch.object(system_info, 'import_string', side_effect=ImportError)
+    def test_installation_method_not_defined(self, *args):
+        self.assertEqual(system_info.get_openwisp_installation_method(), 'unspecified')
+
     def test_log_module_version_changes_on_new_installation(
         self,
     ):